A cylindrically-shaped motor shaft forming a rotational axis of a motor includes a processing target region, a spiral groove, and a reverse spiral groove intersecting with the spiral groove at at least one point. The processing target region has a first end closer to a bottom face of the cylindrical shape and second end closer to a top face side of the cylindrical shape. At least one turn of the spiral groove surrounds an outer circumferential surface in the processing target region. The reverse spiral groove surrounds the outer circumferential surface in the processing target region in a direction reverse to that of the spiral groove.